(a) Field of the Invention
The present invention relates to a driving device of a light source for a display device.
(b) Description of Related Art
Display devices used for monitors of computers and television sets generally include self-emitting display devices such as organic light emitting displays (OLEDs), vacuum fluorescent displays (VFDs), field emission displays (FEDs), and plasma panel displays (PDPs), and non-emitting display devices such as liquid crystal displays (LCDs) requiring external light source.
An LCD includes two panels provided with field-generating electrodes and a liquid crystal (LC) layer having dielectric anisotropy and interposed therebetween. The field-generating electrodes that are supplied with electric voltages generate electric field across the LC layer, and the light transmittance of the liquid crystal layer varies depending on the strength of the applied field, which can be controlled by the applied voltages. Accordingly, desired images are displayed by adjusting the applied voltages.
The light for an LCD is provided by lamps equipped at the LCD or may be a natural light.
When lamps are used, the brightness on a screen of the LCD is usually adjusted by adjusting the ratio of on and off durations of the lamps or adjusting the current flowing through the lamps.
The artificial light source, which is part of a backlight assembly, is often implemented as a plurality of fluorescent lamps such as CCFL (cold cathode fluorescent lamp) and EEFL (external electrode fluorescent lamp) driven by an inverter. The inverter converts a DC voltage into an AC voltage and applies the AC voltage to the lamps to be turned on. The inverter adjusts luminance of the lamps based on a luminance control signal to control the luminance of the LCD. In addition, an inverter feedback controls the voltages applied to the lamps based on the currents of the lamps.
For controlling a plurality of CCFLs, the backlight assembly also includes a plurality of transformers connected to the lamps to apply a high voltage to a hot electrode of each CCFL, and a resistor sensing currents flowing through the CCFLs and connected between cold electrodes of the CCFLs and a ground. The sensed current is relative to the total current flowing through the cold electrode of each CCFL. Thus, the operating states of the CCFLs are not exactly determined in using the sensed current.
In general, as the voltage become higher the leakage current increases. Thus, the leakage current being occurred on the hot electrode is more than that on the cold electrode, thereby the difference between leakage currents on hot electrode and the cold electrode becomes large. The leakage current difference brings an imbalance in luminance of lamps and deterioration of the image quality.
Since a separate transformer is required for controlling each lamp, the volume and the manufacturing cost of the backlight assembly become enlarged.